1. Field of the Invention
The present invention relates to electronics. More specifically, the present invention relates to amplifiers.
2. Description of the Related Art
High dynamic range, low distortion, wideband amplifiers are used in many applications such as pulse mode applications, spectrally pure waveform generation, RF (radio frequency) and IF (intermediate frequency) amplifiers, video processing circuits, and as residue amplifiers for subranging analog to digital converters. These circuits find application in a myriad of fields including communications, high quality audio and video, instrumentation, electronic warfare, radar and sonar.
In these applications, a closed loop amplifier is required to provide accurate, stable, voltage gain. These amplifiers must be linear to better than 0.001% for signal frequencies from DC to IF and higher as the transition frequency fT increases with improvements in processing technologies. Prior art amplifiers do not meet this requirement for high speed, high resolution applications.
A closed loop amplifier is typically comprised of an operational amplifier (op amp) and two resistors connected in a feedback configuration. The closed loop voltage gain G of a closed loop amplifier is given by A/(1+AB), where A is the gain (or open loop gain) of the op amp and B is the feedback ratio of the circuit. For many applications, A can be considered constant since, for large A and small B, small changes in A are attenuated by the loop gain, AB. However, when extremely high accuracies are required, changes in A cannot be ignored. In particular, if A changes as a function of the input, the output will no longer be a linear function of the input and intermodulation distortions will occur.
Unfortunately, the gain A of a conventional op amp will vary as a function of the input. This is due primarily to the Early effect (in bipolar transistors) and channel length modulation (in field-effect transistors, or FETs) in the transistors in the signal path. A typical op amp includes a PMOS current source, which sets the load impedance rO of the gain stage. The voltage across the current source, however, changes as a function of the input voltage. This causes a change in the output impedance rO of the current source due to an effect called channel length modulation. This effect is caused by the transistor's channel length changing as its drain to source voltage changes. This length change in effect changes the output impedance of the FET. Therefore, the output impedance rO of the current source changes as a function of the input voltage. Since rO primarily sets the gain A of the op amp, any variations of rO will vary A. Thus, the gain A varies as a function of the input voltage. Channel length modulation can easily change the gain A of the op amp by 1%.
In a closed loop amplifier, an error in A will be reduced by the loop gain AB. At low frequencies where the loop gain is high (assume 103), a 1% change in A would change the output only approximately 0.001%, which is acceptable. However, at higher frequencies where the gain drops off and the loop gain might only be about 10, a 1% change will result in a 0.8% change in the output. This cannot be tolerated. The use of this amplifier is therefore restricted to a bandwidth that allows sufficient loop gain to reduce the errors due to channel length modulation. Clearly channel length modulation has limited both the bandwidth and the dynamic range of the amplifier.
A similar problem occurs in the output stage of the op amp, which typically includes a Darlington pair. The voltages across the Darlington transistors also vary as a function of the input voltage. Variations in the collector to emitter voltage of a bipolar transistor results in variations in the effective base width of the transistor, causing the transistor's output impedance to become finite. This is known as the Early effect, and it causes a distortion in the gain A of the op amp.
The error due to the Early effect is inside the loop, so for low frequencies it is negligible but for high frequencies where the loop gain approaches 10, this error would add significantly to the distortion products of the amplifier. More limiting to the amplifier's performance is the collector to base capacitance CCB of both Darlington pair transistors. Since their collectors are typically tied to the power supply, the capacitances must charge and discharge as the input voltage changes. This will impact the settling time and distortion of the amplifier due to the additional charging and discharging currents and the settling of those currents the capacitances cause.
Hence, there is a need in the art for an improved amplifier offering faster speed and larger dynamic range than prior art approaches.